FIG. 1 shows a high-bypass gas turbine engine 10. The engine 10 comprises, in axial flow series, an air intake duct 11, an intake fan 12, a bypass duct 13, an intermediate pressure compressor 14, a high pressure compressor 16, a combustor 18, a high pressure turbine 20, an intermediate pressure turbine 22, a low pressure turbine 24 and an exhaust nozzle 25. The fan 12, compressors 14, 16 and turbines 20, 22, 24 all rotate about the major axis of the gas turbine engine 10 and so define the axial direction of gas turbine engine.
Air is drawn through the air intake duct 11 by the intake fan 12 where it is accelerated. A significant portion of the airflow is discharged through the bypass duct 13 generating a corresponding portion of the engine 10 thrust. The remainder is drawn through the intermediate pressure compressor 14 into what is termed the core of the engine 10 where the air is compressed. A further stage of compression takes place in the high pressure compressor 16 before the air is mixed with fuel and burned in the combustor 18. The resulting hot working fluid is discharged through the high pressure turbine 20, the intermediate pressure turbine 22 and the low pressure turbine 24 in series where work is extracted from the working fluid. The work extracted drives the intake fan 12, the intermediate pressure compressor 14 and the high pressure compressor 16 via shafts 26, 28, 30. The working fluid, which has reduced in pressure and temperature, is then expelled through the exhaust nozzle 25 and generates the remaining portion of the engine 10 thrust.
Fuel is provided to the combustor for combustion with the air by a fuel system. A typical prior fuel system includes a main fuel line, a fuel pump (which may be mechanically driven by a shaft or may be driven by an electric motor), and a fuel metering unit (FMU). Fuel is pumped from a fuel tank by the pump through the main fuel line to the fuel metering unit. A portion of the fuel received by the FMU is delivered to a fuel injector located downstream of the FMU within the combustor to provide a metered flow rate of fuel to the engine. The remainder of the fuel is recirculated back to the input of the fuel pump. By varying the proportion of fuel delivered to the combustor compared to the recirculated fuel, the rate of flow of fuel delivered to the combustor can be varied in dependence on the fuel demand, which is determined by an engine control unit (ECU or FADEC).
Induction motors have advantages for use in such applications, in view of their relatively high reliability and low cost. Induction motors comprise a stator having at least one electrical winding, and a rotor. In operation, a magnetic field is induced in the rotor by the magnetic field produced by the stator. This rotor magnetic field interacts with the magnetic field created by the stator winding. The stator electrical winding is supplied with AC electrical power, which creates a rotating magnetic field. The rotating magnetic field produced by the stator windings interacts with the magnetic field produced by the rotor to produce mechanical torque, thereby rotating the rotor.
The speed of AC induction motors can be altered either by altering the frequency of the AC current in the stator, or by changing the voltage of the AC power, or often a combination of the two (known in the art as “V/f control”). AC generators used in modern aircraft engines generally produce an AC current having a frequency which varies in dependence on engine speed, typically varying between 360 and 800 Hz. Consequently, where the fuel pump is driven by an induction motor, the rotational speed of the fuel pump will also change in accordance with engine speed. However, in some cases (such as for acceleration or deceleration of the engine) it is necessary to operate the pump at a speed which does not correlate to the current speed of the engine, to better match supply and demand. Consequently, either power electronics must be used to convert the AC current provided by the engine generators to the required frequency, or the motor speed must be controlled by adjusting the input voltage.
Both of these methods have disadvantages. Power electronics units are relatively bulky and heavy, and may require cooling systems, particularly in view of the large electrical current required to provide the needed torque. Voltage regulation is relatively inefficient, and has a limited range of output speeds for a given electrical AC input frequency.
The present invention describes a fuel system and a gas turbine engine which seeks to overcome some or all of the above problems.